1. Field of the Invention
The invention relates to aircraft automatic flight control systems, particularly with respect to reducing limit cycle oscillations induced in the systems by lost motion hysteresis and lag. The invention is applicable to automatic flight control systems for both commercial and military aircraft with respect to control about the pitch, roll and yaw axes. The invention is applicable to drum and cable as well as hydraulic control surface actuator servos.
2. Description of the Prior Art
Automatic flight control systems are known for positioning the aircraft control surfaces in accordance with a variety of control modes. Hysteresis and lag exist in numerous control channels. For example, in certain aircraft there is significant lost motion between the autopilot servo drum and the elevator surface. When the autopilot is engaged and, for example, the altitude control mode is selected, a slow oscillation may be generated because of the autopilot/hysteresis coupling. In other aircraft, hysteresis effects are exhibited in hydraulic control surface positioning systems and pitch and roll low frequency oscillations may be generated in such aircraft. Low frequency oscillations of the type described are often referred to as "limit cycles." It is appreciated that automatic flight control systems are prevalent with conditions under which limit cycle oscillations may occur. Such oscillations are created when a hysteresis block is embedded in a larger autopilot system where the oscillation criteria are satisfied. For convenience, the invention will be described in terms of the aircraft pitch axis.
In automatic flight control systems for modern jet transports, the servo loop that positions a control surface in response to a control surface position command, is often closed utilizing servo motor position and rate to provide desirable servo bandwidth and stability characteristics. In a particular jet transport, where the autopilot servo drum to elevator surface coupling exhibited significant hysteresis and lag, conventional means of obviating limit cycle oscillation were unsuccessful. The elevator surface position was utilized to compensate for the lost motion in the elevator circuit. In the prior art designs, the elevator surface position signal was substituted for an equal portion of the servo position feedback in the autopilot servo control law. This arrangement was effective to lower the limit cycle amplitudes proportional to the ratio of substitution utilized. In the particular application, a ratio of 80% or higher was required to control the aircraft limit cycle oscillations. The ratio, however, was practically limited to 30% because of the high level of control column activity caused by overcompensating the servo for control system hysteresis and lag.